The present invention pertains to replaceable cartridge type filters and, more particularly, to such a filter utilizing a reverse osmosis cartridge with an assembly for retaining the cartridge in the housing when it is removed for replacement.
Replaceable cartridge filters have been in used in water treatment systems for many years. A typical filter unit of this type utilizes an elongate, generally cylindrical housing or sump which is open at one end for the receipt of a replaceable filter cartridge, and the cartridge and the housing are demountably attachable to an enclosing end cap. The end cap may be a unitary cover or part of a header system for control of fluid flow into and out of the cartridge. The filter cartridge may include any of several well known kinds of filter media, and multiple housings and associated filter cartridges may be attached to a common header system for the serial removal of a wide variety of both suspended and dissolved contaminants from a liquid stream. The media used in filter cartridges varies widely, depending upon the contaminant to be removed, and such media include granular particulate materials, coarse sintered blocks, paper and plastic filters, and semi-permeable membranes.
One type of semi-permeable membrane or reverse osmosis (RO) filter cartridge utilizes a membrane which is spirally wrapped around a porous center tube and enclosed in an impervious cylindrical outer wrap. The opposite ends of the cartridge are open, with one of the ends receiving untreated incoming water under pressure and in which most dissolved solids are separated as the water passes through the semipermeable membrane and flows radially toward the porous center tube. The treated water (or membrane permeate) passes along the center tube to an outlet end. The untreated water containing the concentrated dissolved solids (known as the concentrate and commonly referred to as brine) passes out of the opposite end of the membrane cartridge. Such a cartridge typically includes a brine seal which is interposed between the outer cartridge wrap and the inside wall of the cartridge housing to prevent untreated water from bypassing the cartridge and mixing directly with the brine. However, in certain RO cartridge assemblies, such as an assembly in which the brine seal is located at one end of the element and the outlet for the brine flow is located at the opposite end of the element and housing, a volume of stagnant water is created between the OD of the element and the ID of the housing. This pool of stagnant water is not flushed by continuous flow through the element and therefore provides an area for the propagation of bacteria which, in turn, may lead to fouling by migration to active areas of the RO membrane surface. Depending on how a system is designed, the potential problem of pools of stagnant water, either on the untreated water side or the brine water side of the RO element, is a problem in all residential RO systems which utilize a brine seal.
The product water outlet in the typical RO cartridge comprises a cylindrical neck, which is typically an extension of the center product water tube, and is received in a cylindrical sleeve which forms part of the housing end cap and contains a product water discharge port. The interface between cartridge neck and the sleeve on the cap must be tightly sealed and one typical seal arrangement comprises a pair of axially spaced O-rings which are seated in annular grooves in the cartridge neck and which engage the inside wall of the sleeve in the cap. RO filter cartridges of the foregoing general type are shown, for example, in U.S. Pat. Nos. 4,645,601, 5,002,664, 5,082,557 and 5,266,195.
A typical application for a reverse osmosis membrane filter cartridge of the foregoing general types is in purifying tap water for drinking. As indicated, such a filter cartridge is commonly utilized in a series arrangement with other replaceable cartridge filters which remove from the untreated water other suspended and dissolved solids which cannot be removed by membrane separation. Such water treatment units are commonly mounted beneath a sink on which the tap is located or in another confined and typically somewhat restricted space. A number of problems relating to the replacement of filter cartridges generally and RO filter cartridges particularly have arisen with the use of these systems.
In multi-cartridge systems which typically include three units, manufacturer's like to utilize identical filter housings for simplicity in manufacturing and inventory, as well as to maintain a uniform product appearance. However, certain filter cartridges and often RO membrane cartridges, do not fit well in housings designed for other types of filter cartridges used in these multi-cartridge systems. As a result, special adapters, special covers, or special non-standard housings may have to be used. A specialized and relatively complex housing end cap is shown, for example, in the above identified U.S. Pat. No. 5,082,557. A specialized, non-standard housing is shown in U.S. Pat. No. 5,266,195.
Because RO filter cartridges must handle and provide an interface for three liquid flows, namely, untreated water, treated water, and brine, some means for accommodating the additional liquid flow, not present in other kinds of filter cartridges, must be utilized. In U.S. Pat. No. 5,002,664, the brine flow is accommodated by a special connection through the bottom of the RO cartridge housing. This complicates the construction of the unit, as well as the ease of filter replacement.
Another problem unique to filter cartridges having a neck on one end for the treated water outlet, which includes an O-ring seal arrangement engaging the cylindrical ID of a sleeve in the end cap, is that the tight seal which results causes the filter cartridge to hang up in the end cap when the housing is unscrewed from the end cap for removal and filter cartridge replacement. The filter housing must be slid downwardly along the entire length of the filter cartridge before the cartridge can be independently removed from its sealed attachment to the end cap or header. When operating in a confined space, such as under a kitchen sink, this lack of head space may present a serious obstacle to ease of replacement. This problem is addressed and solved in U.S. Pat. No. 4,645,601 by providing an integral cartridge and housing which are removed together. Such an arrangement, though convenient, is extremely uneconomical because the housing must be replaced each time the filter cartridge is replaced.